This invention relates to impact-resistant polyester molding compounds (molding compositions).
Polyesters are known and proven construction materials, which can be processed, e.g., by injection or extrusion processes. In general, polyesters exhibit good toughness and strength. However, for certain uses, improvements in impact strength and notch impact strength, especially at low temperatures, are desirable.
Certain improvements can be obtained by the use of copolyesters which also contain selected diols or dicarboxylic acids (Angew. Makromol. Chemie 128, 203 ff. (1984)). However, to attain good notch impact strengths at temperatures as low as -40.degree. C., these measures are not satisfactory since modification components must be employed in proportions so high that essential properties of the polyesters are lost.
Another attempt to obtain low-temperature impact-resistant polyester molding compounds is the use of polyester blends, i.e., intimate mixtures of thermoplastic polyesters with tough elastomers or tough, high-molecular weight thermoplastics. Special properties of the added polymers can thus be transferred to the polyester blends without destroying the typical polyester properties.
To obtain polyester blends with satisfactory properties, in each case the polymers to be mixed in should be optimized for the specific use and the specific polyester type. Since such special polymers are not available on the market in the necessary variety, in general there is the problem that in each case special products in mostly small amounts must be produced in a cost-intensive way.
Because of these problems, in practice an alternative approach is taken in the production of polyester-blend polymers which requires the use of olefinic polymers, available in large amounts and many varieties, e.g., polyethylene or ethylene/propylene/(diene) copolymers, which exhibit a high low-temperature or notch strength.
The obviously preferred and most effective embodiment of the alternative approach discussed above consists of grafting of high molecular weight olefinic elastomers, e.g., EP(D)M rubbers or hydrogenated styrene/butadiene block copolymers, with alpha, beta unsaturated carboxyl, carboxy anhydride or carboxylic acid ester monomer derivatives, e.g., maleic acid anhydride, fumaric acid, acrylic acid, methacrylic acid or their esters. Grafting can take place in solution or in the melt with or without addition of radical-forming additives. Such processes are exemplified in U.S. No. 3 882 194 and DE-OS 24 01 149.
All the products that can be used for the alternative method described above must exhibit a high molecular weight and consequently have high melting viscosities, if they are to produce good results in the polyester blends. At the same time optimal properties and efficiency are attained only if the polymers are finely distributed in the polyester matrix. This incorporation causes difficulties because of the high viscosity of the addition polymers and because of the unfavorable polyester/added polymer viscosity relationship. Fluctuations in the flow rate and shearing conditions make the production of a reproducibly good quality difficult. Use of mixing aggregates possessing a high shear strength does reduce the scattering of the values but requires a high energy expenditure and can result in damages to the polyester blends.